The present invention relates to a velocity detector for detecting the rotational speed of a motor or the travel velocity of a moving element.
In a feedback control for positioning or locus control, a stable control requires not only the feedback of position but also the feedback of a velocity signal which is detected by using the signal from a positioning detector such as a rotary encoder or a linear encoder. Recently, in order to perform a highly accurate positioning and locus control, a highly accurate velocity detector has been increasingly required.
With reference to the drawings, an example of the prior art velocity detectors mentioned above is explained hereinafter.
FIG. 3 indicates the arrangement of a prior art velocity detector. In FIG. 3, the elements 100 and 101 are amplifiers for amplifying the signals from an encoder 1000 outputting sinusoidal waves according to the change in position; elements 102 and 104 are wave-form shaping circuits; elements 103 and 105 are differentiators; elements 106 and 107 are multipliers, and element 108 is an amplifier for adding together the signals from the multipliers 106 and 107 and for outputting them.
For the velocity detector arranged as shown above, the operation is explained hereinafter.
The sinusoidal wave signals obtained from the encoder 1000, which are different in phase by 90.degree. from each other, are amplified by the amplifiers 100 and 101, and then are divided into two pairs of system signals, the first pair of which inputted through the waveform shapers 102 and 104 into the multipliers 106 and 107, respectively. The second system signal pair are obtained as an approximate velocity signals by differentiating the position signals from the encoder using the differentiators 103 and 105. In order to use a portion less affected with non-linearity of the velocity signals differentiated by the differentiators 103 and 105 and obtained as a differentiated signal, the second system signals are inputted reversely into the multipliers 106 and 107, changed in polarity according to travel direction by the output from the waveform shapers 102 and 104 previously inputted into the multipliers 106 and 107, and outputted as a velocity signal by adding together the outputs from the amplifiers 106 and 107 using the amplifier 108.
However, in the arrangement mentioned above, a velocity signal is approximately obtained by the differentiated signal of a position signal to accommodate switching, so that the velocity signal varies with the amplitude of the position signal and during high velocity travel, the velocity signal will contain a ripple component.
Also, when detecting a velocity during a short time region within the sinusoidal wave pitch of a position detector or when detecting a low velocity, the detection is affected by the amplitude of the position signal, causing an error of in the detected velocity.